Commercial aviation regulatory agencies have developed required navigation performance (RNP) protocols to facilitate the management of air traffic. Required navigation performance equipped aircraft can safely operate along various routes with less separation than previously needed. This can be significant because less separation means that the number of aircraft that can safely use a particular airspace may increase, and therefore accommodate the increasing demand for air traffic capacity. Under these protocols, RNP values may be assigned to various segments, or legs, of an aircraft's flight plan. For example, during approach an aircraft is typically assigned an RNP value of 0.3 nautical miles (nm). Moreover, for enroute portions of a flight aircraft are typically assigned an RNP value of 2.0 nm, for terminal portions the assigned RNP value is typically 1.0 nm, and when flying over the ocean the RNP value is typically 4.0 or 10.0 nm.
The RNP value defines an airspace within which the aircraft should remain for a predetermined percentage (e.g., 95 percent) of the total flying time. This airspace may be referred to as the RNP Obstacle Evaluation Area or, more simply, the RNP corridor. If the aircraft is RNP capable and if the pilot is appropriately certified, the pilot may attempt to travel the assigned landing leg while remaining within the RNP corridor. If, during the landing attempt, the aircraft breaches an RNP boundary and the leaves the corridor, a warning indicator (e.g., a hazard light) is produced inside the aircraft's cabin and the landing may be aborted and attempted again at a later time.
Closely related to RNP, is what is known as the estimated position uncertainty (EPU). The EPU is basically the value of the total error of the aircraft navigation system. It may thus be appreciated that as long as the EPU is less than the current RNP value for the present airspace, then the aircraft can continue operating in the assigned RNP corridor.
Many conventional aircraft display systems include various means for displaying current RNP and EPU values to a flight crew. These display systems include implementations for displaying the current RNP and EPU values both numerically and non-numerically. For example, many displays, such as the horizontal situation indicator (HSI), display the RNP and EPU values numerically, and some attitude display indicators (ADI) display the RNP and EPU values graphically. In both instances, the RNP and EPU values may not be displayed to the flight crew in a highly intuitive manner.
Hence, it would be desirable to provide a horizontal situation indicator that displays RNP and EPU values non-numerically and in a fairly intuitive manner. The present invention addresses at least this need.